A lot -- them or hate him the calorie counts seen on many restaurant menus across the country will soon be going national. Thanks to a federal law one of the latest weapons in the war on obesity. They're there to help Americans pick healthy choices while dining out. But does all that menu math really add up -- could be packing in more calories. Than you bargained for ABC's Jim model has the straight -- They are supposed to help America's obesity problem. Calorie counts boldly displayed on restaurant menus that's an important that visibility. We needed to states in nine counties require them today and by the middle of this year a federal laws expected to force chain restaurants. Convenience stores and vending machines nationwide. To post calorie counts it's all part of the government's effort to help tighten waist lines and stop the obesity epidemic. How many calories you need every day varies depending on size age and physical activity. But the USDA general recommendations for women are between 1612400. Calories per day. In 2000 to 3000 calories a day for men but how accurate are those calorie counts on the menu. Six. A study by Tufts University sampling food from 42 restaurants says it depends fast food joints are most accurate. Because of the uniform recipes and portion. But -- sometimes wide variations found in sit down restaurant. We found that -- 20% of the foods we tested had a hundred calories or more over what was stated on the menu. And we would consider that to be a considerable amount I know that that may not sound like a lot butts if -- someone -- to -- hundred calories extra per day for a year. They can gain up to ten pounds so that could really be a problem. And most concerning is that a majority of the errors were made on the diet side of the menu is surfaced that people -- trying to manage their weight. Would gravitate toward and. They may be getting more calories than they expect a just completed ABC news sampling found that more than half of the low cal meals we -- Had more calories than listed on the menu we gathered food under the direction of a nationally known last. Sent producers to three cities where major chains to list their calorie counts on the menu. We collected the food as directed never touching it -- their hands froze it. And send it to the lab in Florida for analysis of samples were taken under ten Harrison 24 food samples from four sit down restaurants and one McDonald's. When they arrived at the laboratory there were frozen on burns the food was stored in a lab refrigerator until each could be tested for its calorie count. Then the food -- put each meal into a big blender. -- -- taken on the entire contents. Into law in the moment genius -- Now he considered difference -- started. It's just wants to appease quality ingredients have been together. The results were surprising. McDonald's did the best both the big Mac meal in the premium chicken sandwich tested thirty calories below the menu -- But the sit down restaurants had results sometimes wildly different than advertised. In all only one calorie count was accurate a skinny list just chicken salad sandwich from Cheesecake Factory. Eleven meals had more calories and on the menu and ten had fewer calories the biggest surprise perhaps involved -- original ribs. They tested -- way under the menu promised all three times we bought them but over 600 calories. Other meals -- over by only a few in the over category this on -- had the biggest variants. Cheesecake factory's fish and -- dinner. Was over by a full 420. Calories that's like drinking three cans of sugary soda on top of your meal all of gardens low calorie seafood Modesto was over its calorie count by 120. In one sample of the -- lighter choice Margarita grilled chicken tested at 120 calories more than advertised. Fast food restaurants tended to be more accurate than sit down foods the fast food industry tends to have a more formulate preparation method it's just a matter of warming it up and serving it to consumers. -- that sit down restaurant things are being prepared on the spots. If if by chance some extra -- gets into the pan that can really make a big difference. All the restaurants and their trade association. Say that most calorie counts are as accurate as possible and test extensively to make sure. But they concede there are variations. Mostly -- the portion size and individual restaurant preparation. In the menus -- an actual calories may very. What can you do take control what is put on top -- the on -- asked for everything fattening on the side. Other high eight calorie additions such as cheeses or sauces. Again just try to order those so that you have a little more control over the -- Important information soon to be on every menu because Americans now -- 13 of their meals outside the home. For Nightline I'm Jim Avila in Washington.

This transcript has been automatically generated and may not be 100% accurate.

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